fetch doc: src side of refspec could be full SHA-1

Since a9d34933 ("Merge branch 'fm/fetch-raw-sha1'", 2015-06-01) we
allow to fetch by an object name when the other side accepts such a
request, but we never updated the documentation to match.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pull-fetch-param.txt

@@ -23,9 +23,11 @@ ifdef::git-pull[]
 endif::git-pull[]
 +
 The format of a <refspec> parameter is an optional plus
-`+`, followed by the source ref <src>, followed
+`+`, followed by the source <src>, followed
 by a colon `:`, followed by the destination ref <dst>.
-The colon can be omitted when <dst> is empty.
+The colon can be omitted when <dst> is empty.  <src> is
+typically a ref, but it can also be a fully spelled hex object
+name.
 +
 `tag <tag>` means the same as `refs/tags/<tag>:refs/tags/<tag>`;
 it requests fetching everything up to the given tag.